You wouldn’t know it to look at her, but Christina Hoff Sommers is apparently the kind of speaker whose very presence on college campuses is so alarming that students require advance notice, also known as a trigger warning.

At least, that’s what happened when the American Enterprise Institute scholar spoke this month at Georgetown University and Oberlin College. Campus feminists kicked into high alert, warning students that her lecture on feminism and criticism of the college “rape culture” could make them “feel unsafe.”

Monday’s Oberlin event came with a “safe space” for people to gather during and after her talk. One student warned beforehand that there would be “gatekeeping” to ensure that nobody entered who might be a “toxic, dangerous and/or violent person.”

All this may surprise anyone who hasn’t visited a college campus in the past few years, but for those who have, trigger warnings represent a familiar and increasingly visible part of the university culture.

The heightened focus on sexual assault has campus feminists on the lookout for speakers, lectures and even books that could trigger traumatic memories for survivors and force them to relive the experience.

The warnings allow such students to prepare for sensitive subjects, say advocates, thus increasing their chances of academic success. But critics, including some professors, argue that the practice is rife with problems for academic freedom and free speech.

Not only do trigger warnings threaten to limit the scope of material that can be taught in the classroom, thus degrading the educational experience, but the phenomenon has begun to morph into a kind of academic witch hunt.

“We are currently watching our colleagues receive phone calls from deans and other administrators investigating student complaints that they have included ‘triggering’ material in their courses, with or without warnings,” said a letter last year from seven humanities professors to Inside Higher Ed. “We feel that this movement is already having a chilling effect on our teaching and pedagogy.”

Angus Johnston, a history professor at Hostos Community College in New York City, may be the best-known professor to defend trigger warnings. He includes such a warning on his syllabus.

“History is often ugly. History is often troubling. History is often heartbreaking,” he said.

“As a professor, I have an obligation to my students to raise those difficult subjects, but I also have an obligation to raise them in a way that provokes a productive reckoning with the material,” Mr. Johnston said in a May 2014 op-ed for Inside Higher Ed.

“And that reckoning can only take place if my students know that I understand that this material is not merely academic, that they are coming to it as whole people with a wide range of experiences, and that the journey we’re going on together may at times be painful,” he said.

Ms. Sommers posted an April 15 video on her Factual Feminist blog arguing that trigger warnings are creating “a hostile environment for critical thinking and free expression.” She cited an anonymous professor on the website White Hot Harlots who said “liberal students scare the sh— out of me.”

“All it takes is one slip — not even an outright challenging of their beliefs, but even momentarily exposing them to any uncomfortable thought or imagery — and that’s it, your classroom is triggering, you are insensitive, kids are bringing mattresses to your office hours and there’s a twitter petition out demanding you chop off your hand in repentance,” said the March 19 post.

Despite the watch-your-back atmosphere, no university has formally adopted trigger warnings as a requirement.

Oberlin administrators backed off a proposal last year after an outcry from the faculty, while efforts at Rutgers and the University of California at Santa Barbara have won student support but not administration approval.

Still, that hasn’t stopped students from creating a de facto policy on some campuses, said Greg Lukianoff, president of the Foundation for Individual Rights in Education.

“Whether this becomes a cultural norm or whether it’s imposed from above on campus, it really is a formula for making universities hesitant to teach controversial material, which sort of defeats the point of the university,” said Mr. Lukianoff, who discusses the subject in his book “Freedom From Speech.”

That message was driven home in December by Harvard Law School professor Jeannie Suk, who wrote in The New Yorker that law students have called for trigger warnings in the teaching of rape law.

“One teacher I know was recently asked by a student not to use the word ‘violate’ in class — as in ‘Does this conduct violate the law?’ — because the word was triggering,” Ms. Suk said in her Dec. 15 article. “Some students have even suggested that rape law should not be taught because of its potential to cause distress.”

Mr. Lukianoff said the Harvard case demonstrates how trigger warnings can work against the best interests of rape survivors.

“You need the lawyers trained in the law related to rape in order to help rape victims,” he said. “And the idea that somehow we’re achieving any good by shielding lawyers from the ugly truths of criminal law is crazy.”

Trigger warnings originated on the feminist blogosphere to warn about articles with graphic content about sexual assault. After a few years, however, even some feminists began complaining online about trigger warnings, saying they were overused.

Despite that, trigger warnings somehow made the leap from the Internet to universities, where they have been embraced in particular by feminist groups. At the same time, the Obama administration has put pressure on universities to demonstrate their commitment to curbing campus sexual assault.

“It is probably not coincidental that the call for trigger warnings comes at a time of increased attention to campus violence, especially to sexual assault that is often associated with the widespread abuse of alcohol,” the American Association of University Professors said in a statement calling trigger warnings “infantilizing and anti-intellectual.”

Examples of materials cited by students as potentially triggering include “The Great Gatsby” by F. Scott Fitzgerald, which has been criticized for portraying misogyny and abuse, and such suicide-involved narratives as Virginia Woolf’s “Mrs. Dalloway” and Chinua Achebe’s “Things Fall Apart.”

At Wellesley College, students insisted last year on moving a statue of a man sleepwalking in his underwear, calling it in a petition “a source of apprehension, fear and triggering thoughts,” according to The Boston Globe.

Students at Oberlin who advocated last year for a formal trigger warning policy insisted that it would not impinge on free speech or academic freedom.

“Trigger warnings exist in order to warn readers about sensitive subjects, like sexual violence or war, that could be traumatic to individuals who have had past experiences related to such topics, not to remove these subjects from academic discussion,” said an April 2014 editorial in The Oberlin Review.

Enthusiasm on campus for trigger warnings isn’t going away. Ms. Sommers said students are now asking for trigger warnings on classroom material that deals not just with sexual violence, but also with topics such as colonialism and racism.

Ultimately, the larger problem isn’t with trigger warnings, but with efforts to impose politically correct censorship on campus on the grounds that “it’s a duty by the university to protect and shield students from speech that might make them uncomfortable,” Mr. Lukianoff said.

“You do occasionally get people saying that this is a silly idea that’s going to end up going nowhere. And that’s not what I’m hearing from professors,” he said. “I definitely hear an increase in requests for trigger warnings. So I think this is a story that’s just beginning, not one that’s nearly over.”